It is well known that hydrocarbons may be produced from subterranean formations through a well that has been drilled into a hydrocarbon bearing formation. In many circumstances, it is desirable to then drill one or more additional wellbores (often referred to as “laterals”) outward from the primary wellbore in an effort to increase the productivity of the well or to access additional hydrocarbons in adjacent formations. This can be an effective and economical way to substantially increase the profitability of a well and to increase the overall recovery of fluids from a single, primary well site and surface installation. These lateral wells may extend outwardly from the primary wellbore for substantial distances (e.g. 2000 feet or more) or may be relatively short “drainholes” which extend only a few feet (e.g. 100 feet or less) into the formation.
During the drilling of a well, it is often necessary, for various reasons, to alter, i.e., sidetrack, the direction of the wellbore. The challenge when drilling laterals is being able to drill precisely on target. Drill rigs are expensive and several extra days of rig time may substantially reduce the profitability of drilling additional laterals. Efficiently drilling laterals, which directly and precisely exit the primary wellbore at the desired location within the wellbore first, requires cutting an opening or a window through heavy casing or liner.
A conventional technique for drilling laterals may involve the setting of a kickoff plug, or the like, in a primary wellbore. A kickoff plug may have a length ranging from about 50 to 500 feet, and may comprise a cement composition. The kickoff plug typically is set in the wellbore by lowering a drillstring or open-ended tubing string to the desired depth and pumping a cement composition into the wellbore. The cement composition is allowed to cure to form a plug. After the cement plug has formed, a drillstring may be used to reinitiate drilling operations. The drillstring and drill bit use the plug to drill in a new direction, so as to thereby deflect the drill string and change the direction in which the drilling proceeds. However, the use of kickoff plugs may be problematic due to the prevention of access to further production of fluids from lower portions of the original wellbore because the cement seals the well at the deviation.
Another conventional method of forming a lateral wellbore uses a whipstock which is inserted into the main wellbore and fixed therein. The whipstock is typically a steel structure that includes a concave, slanted surface along its upper portion arranged to direct anything coming down the wellbore toward one side thereof. In particular, the whipstock forms a guide for gradually directing a cutting device from the main wellbore of the well into and through the wall of the existing wellbore where the new lateral wellbore will be formed or cut. Similar to the kick-off plug method, whipstocks are typically permanently installed. A conventional permanently installed whipstock prevents further access to lower formations below the installed whipstock. Furthermore, wells require some amount of workover to remain productive which is prevented to some degree by the installation of a permanent whipstock. Thus, a whipstock which allows access to further formations and/or production below the whipstock is preferred.
While most whipstocks are permanent, removable whipstocks have been developed, but have not been entirely satisfactory as the process of milling and drilling over the whipstock generally destroys or severely damages the whipstock. The process of removing a whipstock requires hooking the whipstock with a latching device that is accessible from above. The inherent topside location of the latching mechanism makes it vulnerable to the damage caused by the milling bit and it is not uncommon to have considerable delays in pulling out the temporary whipstock.
Furthermore, techniques for drilling windows through the side of a cased wellbore become particularly challenging when the production tubing is considerably smaller than the liner. With extra room in the liner, the milling drillbit tends to jump around on the whipstock and create extra damage to the whipstock and to other parts of the liner, slowing down progress and increasing the risk of problems, especially with respect to the recovery and removal of the whipstock. The issue of small production tubing in a large liner occurs, for example, when the window is well above the bottom of the original borehole and the production tubing is sized so to maintain liquid flow with the gaseous components. It is a significant advantage to use the gas in the production fluids to carry the valuable liquids to the surface and large diameter tubing is known to frustrate that benefit by allowing the gaseous components to bypass the liquids and leave them at the bottom of the tubing.